The study included close to 3,000 elderly people in France with memory problems. Some of them took ginkgo biloba and some did not.

Over five years of follow-up, about the same number of people in the two groups got Alzheimer's disease.

The study is published in the journal Lancet Neurology.

An earlier study followed people in the U.S. for an average of six years. It also found no evidence that ginkgo biloba prevents Alzheimer's disease or delays mental problems.

Ginkgo Used to Boost Memory

Ginkgo biloba is widely used and marketed as a memory-boosting drug. Alzheimer's researcher Lon Schneider, MD, says it shouldn't be.

He says close to 10,000 people have participated in studies examining ginkgo biloba's impact on memory and the brain.

"This supplement has been studied as extensively as any drug, with no evidence that it improves memory" or delays problems with thinking, Schneider says. He says if ginkgo biloba were being developed as a drug, it would have been abandoned long ago.

"The drug company response would probably be, 'We're not wasting another dime on this dog,'" he says.

Alzheimer's Risk Same With, Without Ginkgo

The French study included people in their 70s and older. They had complained to their doctors of having problems with memory.

Half of the people took ginkgo biloba twice a day. The other half took identical-looking placebo pills.

Over the next five years, 4% of the ginkgo biloba group and 5% of the placebo group were diagnosed with Alzheimer's disease.

More Study Needed?

In a written statement, study researcher Bruno Vellas of Toulouse, France's Hopital Casselardit called for more studies on whether taking ginkgo biloba over the long term affects age-related memory decline.

"The fact that prevalence of [Alzheimer's disease] is expected to quadruple by 2050 suggests that research into preventative therapies for this disease needs to receive urgent attention," he says in the statement.

But Schneider says investigators would be better off looking elsewhere for effective treatments.

"If there were truly nothing else that might delay the onset of dementia I could understand why users might want to stick with ginkgo biloba," he says. "But we are learning about more and more health behaviors that appear to impact risk."